1. Field of the Invention
The present invention relates to an arrangement for actuating valves employed in an internal combustion engine.
2. Description of the Related Art
A four-valve engine that has two intake valves and two exhaust valves per each cylinder is known in the art. In some of the four-valve engines, the two intake valves for each cylinder are bridged to each other by a valve bridge and the two exhaust valves are also bridged by another valve bridge. Upper surfaces of these valve bridges are pressed by ends of rocker arms respectively such that the two intake valves are forced downwards (or in a valve opening direction) simultaneously and the two exhaust valves are also forced downwards (or in a valve opening direction) simultaneously.
The end of each rocker arm performs a pivot movement about an associated shaft. Thus, the rocker arm end slides on the upper surface of the valve bridge while the rocker arm pushes the valve bridge downwards. This sliding movement of the rocker arm takes place in a certain small area on the valve bridge upper surface. Thus, this area is only subjected to wear. In order to prevent this local wear, Japanese Utility Model Application Laid Open Publication No. 2-126006 provides an improvement. Specifically, a rotatable chip member is located on the valve bridge top surface such that it contacts the rocker arm. Since the chip member rotates, the contact between the rocker arm and chip member varies. Accordingly, the wear of the chip member is less concentrated.
In this improvement, however, the pivot movement of the rocker arm draws an arcuate orbit that passes through a rotation center of the chip member when it slides in contact with the chip member. As a result, the rocker arm cannot apply a sufficiently large rotational moment onto the chip member due to the drag between the rocker arm and chip member. Thus, the chip member does not rotate very much, and the local wear is not prevented as much as expected.
Another known arrangement for actuating valves of an engine is an over head cam arrangement that allows a camshaft to directly contact and drive the intake and exhaust valves. A rotatable lifter is sometimes located at a top of each valve stem such that it contacts the associated cam. By offsetting the lifter center from the cam contact point (press center), the cam can apply a rotational moment to the lifter and causes the lifter to rotate. The drag exerted by the cam in this case positively forces the lifter to rotate so that the wear is not concentrated in a particular area.
This arrangement may be utilized in the engine having rocker arms. Specifically, the rocker arm contact point (press center) may be offset from the chip member rotation center. However, the center lines of the two intake valves (or exhaust valves) lie in the same plane as the chip member center. Thus, if the rocker arm press center were offset relative to this plane, a couple of force would act on the entire valve bridge including the chip member from the rocker arm and valves. This would cause lateral pressure to apply between the valve bridge and intake valves (or exhaust valves). If large lateral pressure were generated, gall would occur.